1. Field of the Invention
This invention relates generally to rotary positioning mechanisms of the class having one or more rotary driven members rotatably positionable by a rotary drive member. The invention relates more particularly to such a rotary positioning mechanism having a selectively adjustable turning ratio which is the turning angle of the driven member(s) per unit turning angle of the drive member. The invention relates also to a novel camera dolly steering mechanism embodying the adjustable ratio positioning mechanism with a single steering shaft cam for selectively steering the dolly in round, conventional, and crab steering modes.
2. Background of the Prior Art
In its broader aspects, this invention relates to a rotary positioning mechanism of the general class having rotary drive and driven members coupled by transmission means in a manner which permits rotary positioning of the driven member(s) by the drive member and adjustment of the transmission turning ratio, i.e., the turning angle of the driven member(s) per unit turning angle of the drive member. Rotary positioning mechanisms of this general class are known and are susceptible of a variety of uses. One of these uses is in a steering mechanism for dollys, such as camera dollys. A more limited aspect of the present invention is concerned with a camera dolly steering mechanism which utilizes the rotary positioning mechanism of the invention to effect selective steering of the dolly in round, crab, and conventional steering modes. The invention will be described in the context of this camera dolly steering application.
Camera dollys are used in the motion picture and television industries as movable supporting platforms for television and motion picture cameras. A typical camera dolly has a frame supported on four steerable wheels and mounting a hydraulically operated boom for supporting the camera. The wheels are connected to a steering shaft or the like for turning of the wheels in their steering movement by turning of the steering shaft. For convenience, the wheel positions in which the wheel planes parallel the longitudinal axis of the dolly, i.e., the wheel rotation axes are normal to the longitudinal axis, such that the dolly is movable along a straight line in the fore and aft direction, are called their neutral positions.
Camera dollys are operable in different steering modes. The improved dolly of this invention, for example, are referred to herein as round, crab, and conventional steering modes. In the conventional steering mode, the front wheels are locked in their neutral positions. The rear wheels are coupled to the steering shaft for steering of the dolly along a straight or curved path by turning of the rear wheels only. When the rear wheels are turned from their neutral positions, their rotation axes intersect the common axis of the front wheels approximately at a common point, such that the dolly moves along an arc having its center at the axis intersection point.
In the crab steering mode, all of the wheels are coupled to the steering shaft for turning in unison on their steering axes in response to turning of the steering shaft in such a way that the wheel rotation axes remain parallel. The dolly is then always movable in a straight line and can be steered to move in any horizontal direction.
Finally, in the circular steering mode, all of the dolly wheels are again coupled to the steering shaft for steering movement by turning of the shaft. In this case all the wheel rotation axes intersect at approximately a common point which moves back and forth along a transverse axis of the dolly passing midway between the front and rear wheels as the wheels are turned by the steering shaft. Dolly movement is then substantially circular about the axis intersection point and, in one position of wheel adjustment, occurs about the geometric center of the dolly wheels.
The prior art is replete with a number of patents covering steering systems for dollys and other vehicles. Examples of such steering systems and other similar systems are found in the following patents:
U.S. Pat. No. 545,165 PA1 U.S. Pat. No. 2,055,747 PA1 U.S. Pat. No. 2,228,247 PA1 U.S. Pat. No. 2,232,015 PA1 U.S. Pat. No. 2,235,044 PA1 U.S. Pat. No. 2,227,458 PA1 U.S. Pat. No. 2,423,266 PA1 U.S. Pat. No. 2,200,736 PA1 U.S. Pat. No. 2,472,442 PA1 U.S. Pat. No. 2,468,801 PA1 U.S. Pat. No. 2,001,647 PA1 U.S. Pat. No. 2,234,888 PA1 U.S. Pat. No. 2,470,496 PA1 U.S. Pat. No. 2,203,810 PA1 Fench Pat. No. 451,813 PA1 U.S. Pat. No. 2,661,672 PA1 U.S. Pat. No. 1,951,147 PA1 U.S. Pat. No. 2,319,880 PA1 U.S. Pat. No. 1,801,043 PA1 U.S. Pat. No. 1,255,775 PA1 U.S. Pat. No. 1,866,393 PA1 U.S. Pat. No. 2,388,692 PA1 U.S. Pat. No. 2,352,797 PA1 U.S. Pat. No. 2,470,496 PA1 U.S. Pat. No. 2,715,534 PA1 U.S. Pat. No. 2,834,605 PA1 U.S. Pat. No. 2,842,376 PA1 U.S. Pat. No. 2,995,380 PA1 U.S. Pat. No. 1,175,312
In general, the dolly steering mechanisms described in these patents and other dolly steering mechanisms of which I am aware are characterized by a common deficiency which this invention addresses. The deficiency referred to resides in their complexity and resulting high cost. This complexity stems, in large part, from the type of steering adjustment which is utilized to shift the steering system between its various steering modes. In one common type of dolly steering mechanism, for example, the dolly steering shaft and wheels are coupled by sprocket chains trained about sprockets on the shaft and wheels. Steering movement of the wheels in the arcuate steering modes (i.e., round and conventional) is accomplished by laterally displacing or adjusting the sprocket chain runs between the wheels and steering shaft during turning of the shaft to turn the wheels. This lateral adjustment of the chain runs is commonly effected by a cam rotatable by the steering shaft and alters the linear travel of the sprocket chains about the wheel sprockets independently of the steering shaft rotation angle and thereby also the turning angles of the wheels per unit turning angle of the steering shaft. The lateral displacement of the sprocket chain runs and resulting variation in wheel turning angles occur in such a way as to achieve the desired steering modes.
In the prior art steering mechanisms of this kind of which I am aware, each steering mode requires its own sprocket chain coupling between the steering shaft and wheels and its own cam(s) for laterally displacing the chain runs to achieve the desired steering mode. As a consequence, these existing dolly steering mechanisms tend to be complex and costly.